This invention relates to laser transmitters and in particular to a means for driving the laser to produce optimum light pulses.
In standard laser transmitters used in lightwave communications, the laser is driven by a combination of dc bias current which is adjusted by some feedback control circuit and a modulation current based on the input data signal (see e.g., U.S. Pat. No. 4,009,385 issued to Sell). In the normal operation, the dc bias current keeps the laser just below its lasing threshold. The modulation current is added to the dc bias so that the laser is turned on when a pulse corresponding to the data input appears. Since the laser is normally off, power consumption is at a minimum. However, at high bit rates, problems exist in achieving well-shaped optical pulses with standard driving circuits. For example, when silicon bipolar transistors are switched on with subnanosecond rise times at high bit rates (i.e., typically 90 megabits/sec or higher), the current transmitted therethrough tends to overshoot the desired steady state value for the initial portion of the modulation pulse. This overshoot is reproduced in the resulting laser pulse. Further, there is an asymmetry in the current rise and fall times which is also reproduced in the laser pulses.
It is therefore a primary object of the invention to provide a means for driving the laser which produces optimum light pulses even at higher bit rates.